Stainless steel and other passive alloys are typically materials with good corrosion resistance, but with relatively poor tribological characteristics, e.g. adhesive wear characteristics. To solve this problem stainless steel and comparable alloys can be surface hardened at low temperature (below 450-550° C.) by dissolution of nitrogen and/or carbon, by which is obtained a zone of so-called expanded austenite or alternatively expanded martensite. This zone is a supersaturated solution of carbon and/or nitrogen in austenite or martensite and is metastable with respect to carbide/nitride formation. Such low temperature processes can be based on gas, plasma or molten salt; gas processes require use of special activation techniques, whereas for plasma and salt bath activation is immediately achieved and no special treatment is necessary. Thereby a surface zone is obtained in the material, which surface zone contains large amounts of nitrogen and/or carbon; this is due to the relatively low process temperature. The material thereby becomes surface hardened and retains its corrosion resistance. Most passive alloys, such as stainless steel, however cannot immediately be solution hardened with nitrogen and/or carbon, since these passive alloys have an impermeable oxide layer, also called the passive layer, which is the reason for the good corrosion characteristics, but which prevents solution of e.g. nitrogen and carbon. Special techniques for removal of this passive layer are therefore required. These techniques are known to the skilled person.
Most employed technological components are used in a machined condition, which means that the material is inhomogeneously cold deformed (plastically deformed). In many applications such cold deformation is desirable from a component-strength-consideration; the component would not work if it did not have the strength increase from the work hardening induced by cold deformation. This creates a big problem if such cold machined components are surface hardened in a low temperature process, so that the surface is changed to expanded austenite or martensite under uptake of nitrogen and/or carbon. The presence of plastic deformation (defects in the microstructure) in the material implies that nitrides and carbides develop easier by reaction of nitrogen and carbon with e.g. chromium (Cr), which is an alloying element in stainless steel. Consequently an amount of Cr is removed from solid solution and bound as chromium nitride/chromium carbide. This implies that the corrosion characteristics are deteriorated because less chromium is available for maintenance of the passive layer. In local areas such Cr-depletion can be pronounced and result in loss of corrosion protection at the surface of the area. The precipitation of nitrides/carbides is called sensitisation. In particular on dissolution of nitrogen this phenomenon is very pronounced, because chromium nitrides are more stable than chromium carbides and can be formed at lower temperature. This means that the temperature at the low-temperature process must be lowered (further) to avoid sensitisation, which is undesirable since the process thereby proceeds more slowly. For extreme degrees of deformation in stainless steel there is perhaps not even a lower limit to sensitisation.
At low-temperature hardening of cold deformed stainless steel workpieces sensitisation will occur in connection with the low-temperature dissolution of nitrogen and/or carbon, which takes place at temperatures below 550° C. To solve the problem with sensitisation in cold deformed materials upon low-temperature surface hardening a full annealing of the components has—where possible—been made by a so-called austenitisation in vacuum or hydrogen atmosphere. Full annealing is a process, which is carried out at temperatures above 1020° C., typically in the range 1020-1120° C. Thereby the cold deformation in the material is annihilated and the low-temperature dissolution can be carried out without the risk of sensitisation. However, the process provides the problem that the strength of the cold-worked metal is reduced—this is referred to as a so-called egg shell effect in the material, i.e. the material becomes soft with a hard thin surface, when the workpiece is subsequently low-temperature hardened. By carrying out an austenitisation the core strength of the material is reduced to that of annealed material, and this process requires that the core strength of the treated component is a design parameter of less importance.
Another possibility is to employ a carburising process where only carbon is dissolved in the material at low temperature, i.e. formation of carbon expanded austenite. Sensitisation is not as critical for carbon dissolution as it is for nitrogen dissolution (nitriding and nitrocarburising) and hence leads to less influence on the corrosion resistance. However, for components with a strong degree of cold deformation even this is considered detrimental. Another disadvantage by only employing carbon dissolution is that a lower surface hardness is obtained than for nitrogen dissolution and that the composition profile (hardness) cannot be adjusted in the same way (see e.g. EP 1095170 B1 and WO 2006/136166 A1).
In e.g. Georgiev et al, Journal of Materials Science and Technology, Vol. 4, 1996, No. 4, pp. 28 and Bashchenko et al, Izvestiya Akademii Nauk SSSR. Metally, no 4, 1985, pp. 173-178, it is shown that nitrogen and/or carbon can be dissolved in stainless steel at high temperature (above about 1050° C.) under equilibrium conditions. It is shown that by employing high temperatures the problem with permeation of the passive layer of stainless steel can be bypassed, since this becomes unstable at these high temperatures. It is also described that the solubility temperature for chromium carbide and chromium nitride lies below this temperature. Consequently, carbides and/or nitrides are not formed at these high temperatures. The solubility of nitrogen/carbon is however relatively limited and for austenitic stainless steels no actual surface hardening occurs; this applies in particular for carbon. To avoid precipitation of carbides/nitrides during cooling a fast cooling rate is required. For martensitic stainless steel types a significant hardening of the surface can take place by fast cooling; however, the hardening effect is at a significantly lower level than obtained by processes for formation of expanded austenite.
WO 2008/124239 suggests a hybrid carburisation process with intermediate rapid quench, according to which a carbon hardened surface in a metal workpiece can be formed without forming carbide precipitates by subjecting the workpiece to both high temperature carburisation and low temperature carburisation, wherein immediately after high-temperature carburisation, the workpiece is rapidly quenched to a temperature below which carbide precipitates form. The rapid quenching may be accomplished using e.g. immersion of the workpiece in water, oil or other cooling medium such as a gas or molten salt. WO 2008/124239 fails to recognize the issues of cold-deformation and formation of carbides and/or nitrides during a subsequent low-temperature hardening.
There is a need for a method which allows low temperature dissolution of nitrogen and/or carbon for hardening of passive alloys such as stainless steel, where the problems with sensitisation and/or adjusting the composition profile are solved.
To overcome the problem with sensitisation in connection with low temperature nitriding and/or carburising of cold deformed workpieces the prior art suggests to anneal the material first, so that partial or full re-crystallisation is obtained; alternatively only a recovery of the material. Thereby the cold deformation in the material, and the strengthening obtained from the cold deformation, is annihilated, but on the other hand the low temperature dissolution can be carried out without problems with sensitisation. However, this solution fails to provide components having high core strength.
The aim of the present invention is to provide a method, which allows the formation of expanded austenite or expanded martensite by low temperature nitriding and/or carburising of products shaped through cold deformation and prepared from passive alloys, in particular stainless steel, without sensitisation occurring in the workpiece and where the strengthening effect obtained is comparable to or possibly even larger than the strengthening effect obtained by cold deformation.